Summary: If you are a fan of the show, be aware that this is a musical where the singers are not always the same people as the voice actors. Also, this movie is NOT CANON.
If you're coming into this movie with fresh eyes having never seen the show, then you'll probably think that it's just okay or interesting, but with perhaps too many songs.
The Good. Animation was amazing and beautiful. A solid upgrade from the show.
The Mediocre. The writing & plot (kinda par for the course though). This movie is not canon and braches completely away from the show. The musical numbers aren't that memorable.
The Bad. Too many songs and musical numbers. This 102 minute movie (including the credits) has 82 minutes of music over 28 different songs. On a personal note, I found myself audibly groaning and saying "Please no, not another one" a few times within the movie.
The Ugly. Different people spoke the lines and sang the songs for some characters. It's most noticable for Ladybug, the main character, and it is a stark contrast in the first 5 minutes of the film that it has actually turned some people away from watching the film entirely, including some die-hard fans of the show.
In all, I'd say for the animation alone there is one extra star, but this film is closer to a 29/100 rating for me, just shy of a 3/10 score.